Picture encoding system conversion device and encoding rate conversion device

ABSTRACT

A picture encoding system conversion device and a code rate conversion device for realizing the conversion taking into account both time delay and picture quality using the information on the code volume of the encoding parameters, input and output buffers and an input bitstream. There are provided a decoder  1  including an input buffer  21 , a VLD unit  22 , an inverse quantizer  23 , an IDCT unit  24 , an adder  35 , a frame memory  26  and a motion compensation prediction unit  27 ; an encoder  2  including an adder  31 , a DCT unit  32 , a quantizer  33 , an inverse quantizer  34 , an IDCT unit  35 , an adder  36 , a frame memory unit  37 , a motion compensation prediction unit  38 , a VLD unit  39  and an output buffer  40 ; and a transcoder controller  3  including a decoder monitor unit  51 , an input buffer monitor unit  52 , a reception transmission channel monitor  53 , a sending transmission channel monitor  63 , an output buffer monitor unit  62  and a quantization step controller  74 . The quantization step controller  74  modifies the quantization step of the encoder based on the information from the input buffer monitor, output buffer monitor, decoder monitor, reception transmission channel monitor and sending transmission channel monitor.

FIELD OF THE INVENTION

[0001] This invention relates to a picture (or image) encoding systemconversion device and encoding rate conversion device and, moreparticularly, to a device which performs rate control taking both thepicture quality and time delay accompanying the conversion into account.

BACKGROUND OF THE INVENTION

[0002] In a digital picture (or image) communication system or a digitalpicture communication service, the technique of encoding the pictureinformation to reduce the information volume to effect transmission andstorage is used in transmitting and storing the picture information. Asan encoding system for moving picture, internationally standardized byITU-T(lnternational Telecommunication Union), there are known anencoding system prescribed by H. 261 Recommendations providing forstandardization of transmitted pictures of visual telephone and videoconference and H. 263 Recommendations providing for standardization ofpictures transmitted over a low bitrate network, such as PHS (PersonalHandy-Phone System).

[0003] In the encoding system for moving pictures, internationallystandardized by ISO (International Organization for Standardization),there is known MPEG (Motion Picture Experts Group) 1, as an encodingsystem for stored video images, while there are known MPEG-2 and MPEG4as a universal encoding system and a low bit rate encoding system,respectively.

[0004] The respective encoding systems used for moving picturecompression are merely analogous to one another to the extent that thesesystems use the DCT (Discrete Cosine Transform), motion compensationprediction and Huffman codes in common, which constitutes only a part.However, bitstreams obtained on actual encoding differ from one systemto another.

[0005] So, if it is desired to interconnect a system employing a certainpicture encoding system to another encoding system employing anotherencoding system, picture codes, once formed, need to be decoded intopicture signals, which then need to be re-encoded as an input picture.

[0006] For example, if the H. 261 bitstream is to be converted into thatof H. 263 or MPEG4, not having the in-loop filtering function, such asone provided in H. 261, it is necessary to decode the encoded bitstreaminto an input Picture and to re-encode the resulting picture signals.

[0007] On the other hand, if it is desired to convert a bitstream of H.263 or MPEG4 into that of H. 261, it is necessary to decode thebitstream into a picture and to re-encode the resulting picture. This isbecause the maximum value of the motion vector of H. 263 or MPEG4 islarger than that of the motion vector of H. 261, while the motion vectorof H. 261 merely has the vector merely of an integer precision.

[0008] That is, for converting a picture encoding system into anotherappreciably different picture encoding system, it is necessary fortransforming a bitstream to interconnect a decoder to an encoder, firstto revert the input bitstream transiently into picture signals by adecoder, and then to re-encode the picture signals as an encoder inputsignal.

[0009] For moving picture data, encoded in accordance with a certainencoding system, there is known an encoding rate converting system, as amethod for realizing rate conversion between an input bitstream and anoutput bitstream. This rate conversion is carried out when the bandwidthof a transmission channel for the input bitstream differs from that oftransmission channel for an output bitstream.

[0010]FIG. 10 shows an embodiment of a structure of a conventionalpicture encoding system converting device. Referring to FIG. 10, thispicture encoding system converting device includes a buffer 5 forstoring a bitstream output from outside, a decoder 6 for decodingpicture codes output from the buffer 5, an encoder 7 for encodingpicture signals output from the decoder 6 and another buffer 8 forstoring picture codes output from the encoder 7 to output the picturecodes to outside.

[0011] The encoder 7 monitors the storage volume (occupied volume) inthe buffer 8. This stored volume in the buffer is utilized forcontrolling the volume of the generated codes during encoding by theencoder 7.

[0012] As this sort of the picture encoding system conversion device,there is proposed in e.g., JP Patent Kokai JP-A-7-107461 a pictureencoding system conversion device adapted for storing the hysteresisinformation of encoding parameters, such as motion vector orquantization step size etc., used in decoding, and referencing theseencoding parameters to determine the encoding parameters to effectre-encoding.

[0013] In e.g., JP Patent Kokai JP-A-7-288804, there is proposed, as are-encoding device for picture signals in which it is possible toprevent picture quality deterioration resulting from re-encoding and tofreely select the picture quality on re-encoding, a configuration inwhich the number of quantization bits is set in addition to theprediction mode, motion vector and the quantization step size, asencoding parameters obtained on decoding an input bitstream, to enablere-encoding with an optional data amount.

[0014] In e.g., JP Patent Kokai JP-A-8-111870, there are also proposed,as a method and apparatus for re-encoding the picture information insuch a manner as to assure optimum picture quality even for a picturehaving encoding hysteresis, a method and apparatus in which re-encodingis achieved using the prediction mode, motion vector, quantization stepsize or periods or phase of picture types, as encoding parametersobtained on decoding the input bitstream.

[0015] In e.g., JP Patent Kokai JP-A-10-32830, there is proposed adevice for re-encoding picture signals in which the quantization stepsize acquired on decoding an input bitstream is used to determine thequantization step size of an encoder to realize the re-encoding.

[0016] In e.g., JP Patent Kokai JP-A-10-336672, there is proposed, in anencoding system conversion device in which, in re-encoding encodedpicture data in accordance with a different encoding system, theprocessing volume is diminished without deteriorating the motion vectordetection accuracy, a configuration in which the motion vector obtainedin decoding encoded picture data is stored and scaled depending on theconversion scale of the picture size, or is converted in quantitydepending on the number of frames, the so scaled or converted motionvectors are provided as candidates and one of these candidates is usedto effect re-encoding.

[0017] In e.g., JP Patent Kokai JP-A-11-285002, there is proposed amoving picture encoding device in which quantization control isperformed on a bitstream obtained on encoding a moving picture so thatthe bitrate of a bitstream acquired on re-encoding will fall within apreset range.

[0018] As another example of the encoding rate conversion device, thereis proposed in e.g., JP Patent Kokai JP-A-8-251587 a configuration of arate conversion device for encoded picture data having a performancecomparable to the transcoding using means simpler than the transcoding.In the above-described encoding rate converting device, disclosed in theJP Patent Kokai JP-A-8-251587, there is proposed a configuration inwhich inverse-quantized encoded data is re-quantized to control the ratecontrol of the quantization level. However, this conventional encodedpicture data rate conversion device, described in the JP Patent KokaiJP-A-8-251587, which decodes the received bitstream and uses theresulting encoding parameters to improve the picture quality onre-encoding, fails to take into account the delay time produced oneffecting conversion of the encoding system.

SUMMARY OF THE DISCLOSURE

[0019] Meanwhile, if the conversion of the picture encoding system is tobe performed off-line, time delay caused in the encoding systemconversion is not so problematic.

[0020] However, in the picture encoding system conversion used inreal-time communication, time delay caused in the encoding systemconversion brings about a lowered service quality.

[0021] On the other hand, the conventional encoding rate conversiondevice, while stating the method for determining the quantization step,fails to take the time delay into account.

[0022] The above-described conventional devices suffer the followingproblems:

[0023] As a first problem, in the conventional picture encoding systemconversion device stated in the above-described JP Patents KokaiJP-A-7-107461, 7-288804. 8-111870, 10-336672 and 10-32830, in which aninput bitstream is decoded and the resulting encoding parameters areused for encoding to convert the encoding system, fails to take the timedelay caused in converting the encoding system into account, thusoccasionally causing marked transmission delay or lowering the encodingefficiency.

[0024] That is, buffer delay caused in an output buffer owned by thepicture encoding system conversion device is not considered so that theproblem of overflow or underf low in the output buffer or the rate ofthe output bitstream is not taken into consideration, thus occasionallyleading to a drastic transmission delay or a significantly loweredencoding efficiency.

[0025] As a second problem, the picture encoding system conversiondevice, described in the above-described JP Patent Kokai JP-A-11-285002,showing means for limiting the range of the bitrates of the outputbitstream to within a preset range, fails to take the realtimeconversion of the encoding system into consideration.

[0026] That is, although the bitrate of the output bitstream is takeninto consideration, no consideration is given to the delay etc. producedin the output buffer.

[0027] As a third problem, although the conventional picture encodingsystem conversion device re-utilizes the encoding parameters produced ondecoding the input bitstream, use of the information on the encodingquantity that can be grasped from the input bitstream is not taken intoaccount. In short, when the input bitstream is decoded, the encodingvolume of each frame, each GOB (group-of-pictures) and each macroblockcan be grasped, however, such encoding volume cannot be utilizedeffectively.

[0028] With a simple encoder, it is possible to calculate an optimumvolume of bit allocation to each frame from the band of the transmissionchannel for transmitting the bitstream and from the picture samplinginterval.

[0029] However, in the case of the picture encoding system conversiondevice, the picture fed to the encoder is a picture acquired on decodingthe input bitstream, and is a picture having a diminished informationvolume. So, the picture quality cannot be expected to be improvedappreciably even if the code volume to be allocated is increased, thusleading to an increased delay time.

[0030] Conversely, if the code volume to be allocated is decreased,there may be an occurrence that the band of the transmission channelcannot be utilized effectively.

[0031] So, it is necessary to make re-encoding using the information onthe code volume as grasped from the input bitstream to take the picturequality, delay time and the band of the transmission channel intoconsideration to effect rate control.

[0032] It is therefore an object according to an aspect of the presentinvention to provide a picture encoding system conversion device and anencoding rate conversion device in which delay time increase or picturequality deterioration may be decreased.

[0033] It is another object according to another aspect of the presentinvention to provide a novel picture encoding system conversion deviceand an encoding rate conversion device, which can make best use of theinformation on the code volume such as that of encoding parameters,input buffer, output buffer, input bitstream or the output bitstream,and in which picture encoding system conversion and code rate conversioncan be realized taking realtime conversion into consideration.

[0034] Further aspects and objects of the present invention will becomeapparent in the entire disclosure.

[0035] According to a first aspect of the present invention, there isprovided a picture encoding system conversion device including a decoderreceiving picture codes, compressed in information volume, from areception side transmission channel via an input buffer and expandingthe received picture codes to output expanded picture codes, an encodercompressing the picture codes, decoded by the decoder, in informationvolume, to generate picture codes to output generated picture codes froman output buffer to a sending side transmission channel, and atranscoder controller controlling the encoder. The transcoder controllerincludes an input buffer monitor monitoring the input buffer of thedecoder, an output buffer monitor monitoring the output buffer of theencoder and quantization step controller modifying (or changing) aquantization step in the compression processing of the encoder based onthe information from the input buffer monitor and the output buffermonitor.

[0036] In the picture encoding system conversion device according to thefirst aspect, the decoder includes a variable length decoder unit, andthe transcoder controller further includes a decoder monitor monitoringthe variable length decoder unit. The quantization step controllermodifies the quantization step of the encoder based on the informationfrom the input buffer monitor, the output buffer monitor and the decodermonitor.

[0037] In the picture encoding system conversion device according to thefirst aspect, the transcoder controller includes a reception sidetransmission channel monitor monitoring the state of the receiving sidetransmission channel, and a sending side transmission channel monitormonitoring the state of the sending side transmission channel. Thequantization step controller modifies the quantization step of theencoder based on the information from the input buffer monitor, outputbuffer monitor, reception side transmission channel monitor and thesending side transmission channel monitor.

[0038] In the picture encoding system conversion device, according tothe present invention, the decoder includes a variable length decodingunit. The transcoder controller further includes a decoder monitormonitoring the variable length decoding unit, a reception sidetransmission channel monitor monitoring the state of the reception sidetransmission channel, and a sending side transmission channel monitormonitoring the state of the sending out side transmission channel. Thequantization step controller modifies the quantization step of theencoder based on the information from the input buffer monitor, outputbuffer monitor, decoder monitor, reception side transmission channelmonitor and the sending side transmission channel monitor.

[0039] In another aspect, the present invention provides a code rateconversion device including a code rate conversion unit receivingpicture codes compressed in information volume from a reception sidetransmission channel by an input buffer, converting the code rate of thepicture codes and subsequently sending out the resultant picture codesthrough an output buffer to a sending side transmission channel, and atranscoder controller controlling the code rate, wherein the transcodercontroller includes an input buffer monitor monitoring the input buffer,an output buffer monitor monitoring the output buffer and a quantizationstep controller modifying the quantization step in the compressionprocessing of the code rate conversion unit based on the informationfrom the input buffer monitor and from the output buffer monitor.

[0040] In the code rate conversion device according to the presentinvention, the code rate conversion unit includes a variable lengthdecoding unit. The transcoder controller includes a decoder monitormonitoring the variable length decoding unit.

[0041] The quantization step controller modifies the quantization stepof the code rate conversion unit based on the information from the inputbuffer monitor, output buffer monitor and the decoder monitor.

[0042] In the code rate conversion device according to the presentinvention, the code rate conversion unit further includes a receptionside transmission channel monitor monitoring the state of the receptionside transmission channel and a sending side transmission channelmonitor monitoring the state of the sending out side transmissionchannel. The quantization step controller modifies the quantization stepof the code rate conversion unit based on the information from the inputbuffer monitor, output buffer monitor, reception side transmissionchannel monitor and the sending side transmission channel monitor.

[0043] In the code rate conversion device according to the presentinvention, the code rate conversion unit includes a variable lengthdecoding unit, and the transcoder controller includes a decoder monitormonitoring the variable length decoding unit, a reception transmissionchannel monitor monitoring the state of the reception side transmissionchannel and sending out transmission channel monitor monitoring thestate of the sending out transmission channel. The quantization stepcontroller modifies the quantization step of the code rate conversionunit based on the information from the input buffer monitor, outputbuffer monitor, decoder monitor, reception side transmission channelmonitor and the sending out side transmission channel monitor.

[0044] Further aspects and modes of the present invention will becomeapparent from the entire disclosure including the following descriptionand the claims. The features of the appended claims are hereinincorporated by reference thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1 shows the structure of a first embodiment of the presentinvention.

[0046]FIG. 2 shows the structure of a second embodiment of the presentinvention.

[0047]FIG. 3 shows the structure of a third embodiment of the presentinvention.

[0048]FIG. 4 shows the structure of a fourth embodiment of the presentinvention.

[0049]FIG. 5 shows the structure of a fifth embodiment of the presentinvention.

[0050]FIG. 6 shows the structure of a sixth embodiment of the presentinvention.

[0051]FIG. 7 shows the structure of a seventh embodiment of the presentinvention.

[0052]FIG. 8 shows the structure of an eighth embodiment of the presentinvention.

[0053]FIG. 9 is a flow diagram showing an embodiment for determining thequantization step.

[0054]FIG. 10 shows the structure of a conventional picture encodingsystem conversion device.

PREFERRED EMBODIMENTS OF THE INVENTION

[0055] In the following, preferred embodiments of the present inventionare explained. Referring to FIG. 1, a picture encoding system conversiondevice according to a first embodiment of the present invention includesa decoder 1 for receiving picture codes, compressed in informationvolume, from a reception side transmission channel via an input buffer21 and for expanding the received picture codes to output expandedpicture codes. An encoder 2 compresses the picture signals, decoded bysaid decoder, in information volume, to generate picture codes to outputthe generated picture codes from an output buffer 40 to a sending outside transmission channel. A transcoder controller 3 includes inputbuffer monitor 52 for monitoring said input buffer 21 of the decoder 1and output buffer monitor 62 for monitoring the output buffer of theencoder 2. There is also provided a quantization step controller 71 formodifying the quantization step in the compression processing of theencoder 2 based on the information from said input buffer monitor 52 andthe output buffer monitor 62.

[0056] Referring to FIG. 2, a code rate conversion device of a secondembodiment of the present invention includes a decoder 1 for receivingpicture codes compressed in information volume from a reception sidetransmission channel by an input buffer 21 and for subsequently sendingout the decoded picture codes through an output buffer 40 to a sendingout side transmission channel. A transcoder controller 3 includes adecoder monitor 51 for monitoring the VLD unit 22 of the decoder 1, aninput buffer monitor 52 for monitoring the input buffer 21 of thedecoder 1, and an output buffer monitor 62 for monitoring the outputbuffer 40 of the encoder 3. There is also provided a quantization stepcontroller 72 for modifying the quantization step in the compressionprocessing of the code rate conversion unit based on the informationfrom the input buffer monitor and from the output buffer monitor.

[0057] Referring to FIG. 3, an encoding system conversion device forconverting a signal encoded in one encoding system into a signal ofanother encoding system, according to a third embodiment of the presentinvention, includes a decoder 1 for receiving a compression-coded signalin an input buffer 21 to decode the signal in a variable length decoder22 to output picture signals. An encoder 2 is fed with a signal outputfrom the decoder 1 to compress the information to output the informationto a sending out transmission channel via an output buffer 40. Atranscoder controller 3 has an input buffer monitor 52 for monitoringthe input buffer 21 of the decoder 1, an output buffer monitor 62 formonitoring the output buffer 40 of the encoder 2, a receptiontransmission channel monitor 53 for monitoring the state of thereception transmission channel and a sending out transmission channelmonitor 63 for monitoring the state of the sending out transmissionchannel. There is also provided a quantization step controller 73 forvariably controlling the quantization step in the quantizer 33 of theencoder 2 based on the information from the input buffer monitor 52,output buffer monitor 62, reception transmission channel monitor 53 andthe sending out transmission channel monitor 63.

[0058] Referring to FIG. 4, an encoding system conversion device forconverting a signal encoded in one encoding system into a signal ofanother encoding system, according to a fourth embodiment of the presentinvention, includes a decoder 1 fed with a compression-coded signal inan input buffer 21 to decode the signal in a variable length decodingunit 22 to output the resulting picture signals. An encoder 2 is fedwith a signal output from the decoder 1 to compress the information tooutput the information to a sending out transmission channel via anoutput buffer 40. A transcoder controller 3 has a decoder monitor 51 formonitoring the VLD unit 22 of the decoder 1, an input buffer monitor 52for monitoring the input buffer 21 of the decoder 1, an output buffermonitor 62 for monitoring the output buffer 40 of the encoder 2, areception transmission channel monitor 53 for monitoring the state ofthe reception transmission channel and a sending out transmissionchannel monitor 63 for monitoring the state of the sending outtransmission channel. There is also provided a quantization stepcontroller 74 for variably controlling the quantization step in thequantizer 33 of the encoder 2 based on the information from the decodermonitor 51, input buffer monitor 52, output buffer monitor 62, receptiontransmission channel monitor 53 and the sending out transmission channelmonitor 63.

[0059] The decoder 1 includes an input buffer 21 fed with a signal fromthe reception side transmission channel, a VLD unit 22 for decoding theencoded data from the input buffer 21, a first inverse quantizer (IQ) 23for inverse quantizing the quantized transform coefficients from the VLDunit 22, an IDCT unit 24 fed with an output of the first inversequantizer 23, a first adder 25 fed at its one input with an output ofthe IDCT unit 24 and a first motion compensation prediction unit 27 fedwith the encoding parameters from the VLD unit 22 and with the output ofthe VLD unit 22. The encoder 2 includes a second adder 31 fed with anoutput of the first adder 25, a DCT unit 32 for transforming an outputof the second adder 31, a quantizer (Q) 33 for quantizing an output ofthe DCT unit 32, a VLC 39 for encoding an output of the quantizer 33, anoutput buffer 40 fed with an output of the quantizer 33 to output to thesending out transmission channel, an inverse quantizer 34 for inversequantizing an output of the quantizer 33, an IDCT unit 35 fortransforming an output of the inverse quantizer 34, a third adder 36 fedwith an output of the IDCT unit 35, a second frame memory unit 37 fedwith an output of the third adder 36 and a second motion compensationprediction unit 38 fed with picture signals from the second frame memoryunit 37 and with encoding parameters from the first motion compensationprediction unit 27. An output 203 from the second motion compensationprediction unit 38 is fed to the VLC 39 and to the second adder 31,while being sent via a switch 41 to the third adder 36. The switch 41 isoff and on for the I-picture (intra-coded picture) and for the P-picture(predictive picture) and for the b-picture (bidirectional predictivepicture), respectively. The second adder 31 subtracts0 from an output ofthe first adder 25 if the picture is an I-picture, while outputting adifference between the output the first adder 25 and the output of thesecond motion compensation prediction unit 38 if the picture is a P- orB-picture.

[0060] Referring to FIG. 5, a code rate conversion device for convertinga signal encoded in one encoding system into a signal of anotherencoding system according to a fifth embodiment of the present inventionincludes a code rate conversion unit 4 including an input buffer 21 fedfrom a reception side transmission channel with a signal compressed ininformation volume, and configured for sending out the picture codescompressed in information volume from an output buffer 40 to a sendingout side transmission channel, and a transcoder controller 3 includingan input buffer monitor 52 for monitoring the input buffer 21, an outputbuffer monitor 62 for monitoring the output buffer 40, and aquantization step controller 71 for variably controlling thequantization step in the quantizer (Q) 82 of the code rate conversiondevice 4 based on the information from the input buffer monitor 52 andthe output buffer monitor 62.

[0061] Referring to FIG. 6, a code rate conversion device for convertinga signal encoded in one encoding system into a signal of anotherencoding system according to a sixth embodiment of the present inventionincludes, in addition to the structure shown in the above-describedfifth embodiment, a decoder monitor 51 owned by the transcodercontroller 3 for monitoring the VLD unit 22 of the code rate conversiondevice 4. The quantization step controller 72 variably controls thequantization step of the code rate conversion device 4 based on theinformation from the input buffer monitor 52, output buffer monitor 62and the decoder monitor 51.

[0062] Referring to FIG. 7, a code rate conversion device for convertinga signal encoded in one encoding system into a signal of anotherencoding system according to a seventh embodiment of the presentinvention includes, in addition to the structure shown in theabove-described fifth embodiment, a reception transmission channelmonitor 53 for monitoring the state of the reception side transmissionchannel and a sending out transmission channel monitor 63 for monitoringowned by the transcoder controller 3 for monitoring the state of thesending out side transmission channel, with both monitors being owned bythe transcoder controller 3. The quantization step controller 72variably controls the quantization step of the code rate conversiondevice 4 based on the information from the input buffer monitor 52,output buffer monitor 62 and the decoder monitor 51.

[0063] Referring to FIG. 8, a code rate conversion device for convertinga signal encoded in one encoding system into a signal of anotherencoding system according to an eighth th embodiment of the presentinvention includes, in addition to the structure shown in theabove-described fifth embodiment, a decoder monitor 51 owned by thetranscoder controller 3 for monitoring the VLD unit, a receptiontransmission channel monitor 53 for monitoring the state of thereception side transmission channel and a sending out transmissionchannel monitor 63 for monitoring the state of the sending out sidetransmission channel, with both monitors being owned by the transcodercontroller 3. The quantization step controller 74 variably controls thequantization step (Q) of the code rate conversion device 4 based on theinformation from the input buffer monitor 52, output buffer monitor 62,decoder monitor 51, reception transmission channel monitor 53 and thesending out transmission channel monitor 63.

[0064] In an preferred embodiment of the present invention, a picturecode rate conversion device includes a code rate conversion unit 4. Thecode rate conversion unit 4 includes an input buffer unit 21 for beingfed with a signal from a reception side transmission channel, a variablelength decoding unit (VLD) 22 for decoding picture codes of said inputbuffer unit 21, a first inverse quantizer (IQ) 23 for quantizing anoutput of the first variable length decoder 22, a first adder 81 on oneinput end of which an output of the first inverse quantizer 23 is fed, aquantizer (Q) 82 for quantizing an output of the first a adder 81, avariable length encoder (VLC) 83 for encoding an output of the quantizer82 to output an encoded output, an output buffer 40 fed with an encodedsignal output of the variable length encoder 83 to output a resultingoutput to a sending out side transmission channel, a second inversequantizer (IQ) 84 for inverse quantizing an output of the quantizer (Q),a second adder 85 for subtracting an output of the first adder 81 froman output of the second inverse quantizer 84, an IDCT unit 86 fed withan output of the second adder 85 as an input, a frame memory unit 87 fedwith and storing an output of the IDCT unit 86, a difference calculatingunit 88 for taking a difference between the current picture output fromsaid variable decoding unit 87 and a picture one frame before from theframe memory unit 87 and a DCT unit 89 fed with an output of thedifference calculating unit 88 as an input. The first adder 81 outputs avalue corresponding to the output of the first inverse quantizer (IQ) 23minus the output of the DCT unit 89. The encoding parameters output fromthe VLD unit 22 are input to the VLC 83. The quantization stepcontroller 74 of the transcoder controller 3 variably controls thequantization step of the quantizer (Q) 82.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] In the following, certain preferred embodiments of the presentinvention are explained. FIG. 1 shows the structure of a firstembodiment of the present invent ion. Referring to FIG. 1, the firstembodiment of the present invention includes a decoder 1, an encoder 2and a transcoder controller 3. The decoder 1 includes an input buffer21, a variable length decoder (VLD) 22, an inverse quantizer (IQ) 23, aninverse discrete cosine transform (IDCT) unit 24, an adder 25, a framememory 26 and a motion compensation prediction unit 27.

[0066] The encoder 2 includes an adder 31, a DCT (discrete cosinetransform) unit 32 a quantizer (Q) 33, an inverse quantizer (IQ) unit34, an inverse discrete cosine transform (IDCT) unit 35, an adder 36, aframe memory unit 37, a motion compensation prediction unit 38, avariable length encoder (VLC) 39, and an output buffer 40.

[0067] The transcoder controller 3 includes an input buffer monitor 52,an output buffer monitor 62, and a quantization step controller 71.

[0068] The decoder 1 and the encoder 2 decodes and encodes,respectively, in accordance with pre-existing encoding systems, such asH. 261, H. 263, or MPEG1, 2 or 4.

[0069] Referring to FIG. 1, the operation of the first embodiment of thepresent invention is explained in detail. First, the operation of thedecoder 1 is explained.

[0070] The input buffer 21 stores a bitstream fed from an externaltransmission channel to output a stored bitstream to the VLD 22, whileoutputting the information 101 stored in the input buffer to the inputbuffer monitor 52.

[0071] The VLD unit 22 entropy-decodes the output bitstream from theinput buffer 21, such as by variable length decoding or run-lengthdecoding, to output the decoded quantized transform coefficients to theIQ unit 23. On the other hand, the VLD 22 outputs encoding parameters201, such as motion vector or the prediction mode, to the motioncompensation prediction unit 27.

[0072] The IQ unit 23 inverse-quantizes quantized transformcoefficients, output from the VLD unit 22, to output theinverse-quantized transform coefficients to the IDCT unit 24.

[0073] The IDCT unit 24 Performs matrix calculations of inverse discretetransform on the transform coefficients output from the IQ unit 23 tooutput the transformed picture signals to the adder 25.

[0074] The adder 25 sums a signal from the IDCT unit 24 and a picturesignal output from the motion compensation prediction unit 27 as laterexplained to output a sum signal to the frame memory 26 and to an adder31 of the encoder 2.

[0075] The frame memory 26 saves the picture signals output from theadder 25.

[0076] The motion compensation prediction unit 27 performs motioncompensation prediction, using the encoding parameters 201 output fromthe VLD unit 22 and the picture signals stored in the frame memory 26,to output the resulting picture signals to the adder 25.

[0077] On the other hand, encoding parameters 202 are output to themotion compensation prediction unit 38 of the encoder 2.

[0078] The operation of the encoder 2 is now explained.

[0079] If the picture is a P- or B-picture, the adder 31 outputs picturesignals corresponding to the sum of the picture signals output from theadder 25 and a prediction signal output from the motion compensationprediction unit 38, whereas, if the picture is an I-picture, the adder31 directly outputs picture signals output from the adder 25.

[0080] The DCT unit 32 executes DCT matrix calculations (processing) oninput picture signals to output the transform coefficients to thequantizer (Q) 33.

[0081] The quantizer 33 performs quantization calculations on transformcoefficients output from the DCT unit 32 to output the resultingquantized transform coefficients to the VLC unit 39 and to the inversequantizer 34.

[0082] It is noted that quantization characteristics are determined bythe quantization step information 103 output from the quantization stepcontroller 71.

[0083] The inverse-quantizer (IQ) 34 inverse-quantizes quantizedtransform coefficients output from the quantizer (Q) 33 to output theresulting transform coefficients to the IDCT unit 35.

[0084] The IDCT unit 35 performs IDCT matrix calculations on thetransform coefficients output from the inverse-quantizer 34 to outputpicture signals corresponding to predicted error signals and thosecorresponding to the encoded picture signals to the adder 36 in case ofthe P- and B-pictures and in case of the I-picture, respectively.

[0085] The adder 36 outputs a signal corresponding to a sum of thepicture signals output from the IDCT unit 35 and the predicted errorsignals in case of P- and B-pictures, whereas outputting the picturesignals unchanged to the frame memory unit 37 in case of I-picture.

[0086] The frame memory unit 37 stores the picture signals output fromthe adder 36.

[0087] Using the encoding parameters 202 output from the motioncompensation prediction unit 27 and the picture signals saved in theframe memory 37, the motion compensation prediction unit 38 performsmotion detection and motion compensation prediction to generate motioncompensated prediction picture signals which are output to the adders 31and 36. The encoding parameters 203 are also output to the VLC unit 39.

[0088] The VLC unit 39 applies entropy encoding, such as VLC orrun-length coding, to the quantized transform coefficients output fromthe quantizer 33 and the encoding parameters 203 output from the motioncompensation prediction unit 38 to output the resulting encoded signalsto the output buffer 40.

[0089] The output buffer 40 stores the encoded signals output from theVLC unit 39 to send the encoded signals to outside the apparatus.

[0090] The operation of the transcoder controller 3 is now explained.

[0091] The input buffer monitor 52 monitors the storage volume in theinput buffer 21 of the decoder 1 to output the input buffer information107 to the quantization step controller 71.

[0092] The output buffer monitor 62 monitors the stored output buffervolume of the output buffer 40 in the encoder 2 to output the outputbuffer information 109 to the quantization step controller 71.

[0093] Based on the input buffer information 107 and on the outputbuffer information 109, the quantization step controller 71 of thetranscoder controller 3 determines the quantization step information 103which is delivered to the quantizer (Q) 33 of the encoder 2.

[0094] The quantization step controller 71 continually monitors theoutput buffer information 109 and, should there be produced overflow inthe output buffer 40, the quantization step information 103 is modifiedto diminish the code volume produced in the encoder 2 to preventoverflow from occurring in the output buffer 40.

[0095] Should there be produced or expected underflow in the outputbuffer 40, the quantization step information 103 is modified to increasethe volume of codes produced in the encoder 2 to prevent the underflowin the output buffer 40. This evades the output buffer 40 from beingdestroyed or running into failure or.

[0096] It is also possible for the quantization step controller 71 toset the target storage volume in the output buffer 40 to determine thequantization step information 103 based on the relative magnitude of theoutput buffer information 109 as the actual storage volume and thetarget storage volume.

[0097] If the actual storage volume is larger than the target storagevolume, the quantization step information 103 is modified to decreasethe volume of the codes generated in the encoder 2, that is forincreasing the quantization step.

[0098] If, conversely, the storage volume is smaller than the targetstorage volume, the quantization step information 103 is modified toincrease the code volume generated in the encoder 2, that is to decreasethe quantization step.

[0099] The target value can be variably set instead of being a fixedvalue. It is also possible to set the upper and lower limit targetvalues and to set the target value within a certain preset range.

[0100] The quantization step controller 71 is able to calculate the codevolume per processing unit from the input buffer information 107 and theoutput buffer information 109 in effecting picture coding systemconversion.

[0101] Now considering the processing unit based pre- andpost-conversion code volumes in case of performing the picture encodingsystem conversion, if the pre-conversion code volume is larger than thepost-conversion code volume, the band of the transmission channeltransmitting the bitstream would be not fully utilized. If, conversely,the post-conversion code volume is larger than the pre-conversion codevolume, the bitstream cannot be sent with the band of the transmissionchannel sending out the bitstream, thus causing delay at the outputbuffer.

[0102] That is, if the pre-conversion code volume is coincident with thepost-conversion code volume, the most effective utilization of the bandof the transmission channel and the time delay are reduced, provided,however, that this is applicable to a case where the band of thetransmission channel for the input bitstream is the same as that of thetransmission channel for the output bitstream.

[0103] If both the bands are different, the post-conversion code volumeis preferably coincident with a pre-conversion code volume multipliedwith a ratio R which is a ratio between the input transmission channelband and the output transmission channel band, e.g., a ratio R of theinput band to the output band.

[0104] Thus, with the quantization step controller 71, effectiveutilization of the transmission channel and reduction in the time delaymay be realized by determining the quantization step depending on thedifference between the processing unit based pre-conversion code volumeand the post-conversion code volume in converting the picture encodingsystem.

[0105] If, for example, the pre-conversion code volume is larger thanthe post-conversion code volume, the quantization step information 103is modified (or modulated) to increase the code volume generated in theencoder 2 at the next processing.

[0106] If, conversely, the pre-convers ion code volume is less than thepost-conversion code volume, the quantization step information 103 ismodified (or modulated) to decrease the code volume generated in theencoder 2 at the next processing.

[0107] In this manner, it becomes possible to determine the processingunit based quantization step in converting the picture encoding system.

[0108] As a method for converting the picture encoding system, it ispossible to use any of the quantization step modifying systems, such H.261, H. 263, or MPEG 1, 2 or 4, which belong to the conventional pictureencoding systems.

[0109] A second embodiment of the present invention is hereinafterexplained. FIG. 2 shows the structure of the second embodiment.Referring to FIG. 2, the second embodiment of the present inventionincludes, in addition to the components of the first embodiment, adecoder monitor 51, owned by the transcoder controller 3, for monitoringthe VLC unit 22. A quantization step controller 72 determines thequantization step information using the input buffer monitor 52, outputbuffer monitor 62 and the decoder monitor 51.

[0110] That is, referring to FIG. 2, the present second embodimentincludes a decoder 1, an encoder 2 and a transcoder controller 3. Thedecoder 1 is made up of an input buffer 21, a VLD unit 22, aninverse-quantizer 23, an IDCT unit 24, an adder 25, a frame memory 26and a motion compensation prediction unit 27. The encoder 2 is made upof an adder 31, a DCT unit 32, a quantizer 33, an inverse-quantizer 34,an IDCT unit 35, an adder 36, a frame memory unit 37, a motioncompensation prediction unit 38, a VLC unit 39 and an output buffer 40.The a transcoder controller 3 is made up of an input buffer monitor 52,an output buffer monitor 62 and a quantization step controller 72.

[0111] Referring to FIG. 2, the operation of the second embodiment ofthe present invention is explained in detail.

[0112] In addition to the operation of the above-described firstembodiment, the operation of the decoding unit 1 includes the operationof the VLD unit 22 outputting encoding parameters 102 to the decodermonitor 51 of the transcoder controller 3. The operation of the encoder2 is similar to that of the previous first embodiment.

[0113] The operation of the transcoder controller 3 is now explained.

[0114] The decoder monitor 51 monitors the VLD unit 22 of the decoder 1and outputs encoding parameters 108 to the quantization step controller72.

[0115] The input buffer monitor 52 monitors the stored input buffervolume in the input buffer 21 to output the input buffer information 107to the quantization step controller 72.

[0116] The output buffer monitor 62 monitors the stored output buffervolume in the output buffer 40 of the encoder 2 to output the outputbuffer information 109 to the quantization step controller 72.

[0117] The transcoder controller 3 determines the quantization stepinformation 103 based on the input buffer information 107 and the outputbuffer information 109 to deliver the so-determined information to thequantizer (Q) 33 of the encoder 2.

[0118] The transcoder controller 3 is also able to determine thequantization step information 103 by referencing also the encodingparameters 108.

[0119] The quantization step controller 72 monitors the output bufferinformation 109, such that, if overflow would be produced in the outputbuffer 40, the quantization step controller 72 causes the quantizationstep information 103 to decrease the code volume generated in theencoder 2 to prevent overflow from occurring in the output buffer 40.

[0120] If underflow would be produced in the output buffer 40, thequantization step information 103 is modified to increase the volume ofcodes generated in the encoder 2 to prevent underflow of the outputbuffer 40. This prevents the output buffer 40 from failure.

[0121] It is possible for the quantization step controller 72 to set thetarget storage volume in the output buffer 40 to determine thequantization step information 103 based on the relative magnitudesbetween the output buffer information 109 as the actual storage volumeand the target storage volume.

[0122] If the actual storage volume would become or is larger than thetarget storage volume, the quantization step information 103 is modifiedto decrease the volume of the codes generated in the encoder 2, that isto increase (or enlarge) the quantization step.

[0123] If, conversely, the actual storage volume is smaller than thetarget storage volume, the quantization step information 103 is modifiedto increase the volume of the codes generated in the encoder 2, that isto decrease (or shorten) the quantization step.

[0124] The target value can be variably set instead of being a fixedvalue. It is also possible to set the upper and lower limit targetvalues and to set the target value within a certain preset range.

[0125] The quantization step controller 72 calculates the code volumeper processing unit based on the input buffer information 107 and theoutput buffer information 109 in effecting picture coding systemconversion.

[0126] Now considering the processing unit based pre- andpost-conversion code volumes in case of performing the picture encodingsystem conversion, if the pre-conversion code volume is larger than thepost-conversion code volume, the band of the transmission channeltransmitting the bitstream is not fully utilized. If, conversely, thepre-conversion code volume is smaller than the post-conversion codevolume, the bitstream cannot be sent with the band of the transmissionchannel sending out the bitstream, thus causing delay at the outputbuffer.

[0127] That is, if the pre-conversion code volume is coincident with thepost-conversion code volume, the effective utilization of the band ofthe transmission channel is achieved, and time delay is reduced,provided, however, that this is appticable to a case where the band ofthe transmission channel for the input bitstream is the same as that ofthe transmission channel for the output bitstream.

[0128] That is, if the bands would differ, the post-conversion codevolume is desirably coincident with the pre-conversion code volumemultiplied with a ratio R which is a ratio between the inputtransmission channel band and the output transmission channel band.

[0129] In the transcoder controller 3, effective utilization of thetransmission channel band and reduction in the delay time can berealized by determining the quantization step depending on thedifference between the processing-unit-based pre-conversion code volumeand the processing-unit-based post-conversion code volume as at the timeof performing the picture encoding system conversion.

[0130] For example, if the pre-conversion code volume would become or islarger than the post-conversion code volume, the quantization stepinformation 103 is modified such as to increase the code volumegenerated in the encoder 2 at the time of the next processing. If,conversely, the pre-convers ion code volume would become or is smallerthan the post-conversion code volume, the quantization step information103 is modified such as to decrease the code volume generated in theencoder 2 at the time of the next processing. This allows to determinethe quantization step per processing unit in converting the pictureencoding system.

[0131] In the second embodiment of the present invention, thequantization step information 103 is determined (additionally) using theencoding parameters 108 output from the decoder monitor at the time ofdetermining the quantization step. The reason is that, taking thepicture quality following the conversion of the picture encoding systeminto consideration, the pre-conversion code volume can be equated to thepost-conversion code volume by performing the encoding using thequantization step obtained in the decoder 1 in performing thequantization in the encoder 2.

[0132] If a quantization step used in decoding significantly differsfrom a quantization step used in encoding, picture distortion tends tobe produced.

[0133] The picture quality can be improved by determining thequantization step information 103 with the aid of the encodingparameters 108 output from the decoder monitor 51. Control may bemanaged by performing fine adjustment, depending on the output buffer40, pre-conversion code volume and on the post-conversion code volume,using the quantization step output from the decoder monitor 51 as aninitial value.

[0134] As the method for modifying the quantization step, use may bemade of any of conventional picture encoding systems, such as thequantization step modifying systems, e.g., H. 261, H. 263, or MPEG1, 2or 4.

[0135] A third embodiment of the present invention is now explained.FIG. 3 shows the configuration of the third embodiment of the presentinvention. Referring to FIG. 3, the third embodiment of the presentinvention includes, in addition to the components of the firstembodiment, a reception side transmission channel monitor (“receptionchannel monitor”) 53 for monitoring the reception side transmissionchannel and a sending out side transmission channel monitor 63 formonitoring the sending out side transmission channel, these monitors 53,63 being provided in the transcoder controller 3. The quantization stepcontroller 73 determines the quantization step information using theinformation output from the input buffer monitor 52, output buffermonitor 62, reception side transmission channel monitor 53 and from thesending out side transmission channel monitor 63.

[0136] That is, referring to FIG. 3, the present third embodimentincludes decoder 1, encoder 2 and transcoder controller 3. The decoder 1is made up of an input buffer 21, a VLD unit 22, an inverse-quantizer23, an IDCT unit 24, an adder 25, a frame memory 26 and a motioncompensation prediction unit 27. The encoder 2 is made up of an adder31, a DCT unit 32, a quantizer 33, an inverse-quantizer 34, an IDCT unit35, an adder 36, a frame memory unit 37, a motion compensationprediction unit 38, a VLC unit 39 and an output buffer 40. Thetranscoder controller 3 is made up of an input buffer monitor 52, areception side transmission channel monitor 53, an output buffer monitor62, a sending out side transmission channel monitor 63, and aquantization step controller 73.

[0137] Referring to FIG. 3, the operation of the third embodiment of thepresent invention is explained in detail.

[0138] The description for the operation of the decoder 1 and theencoder 2 is omitted because it is the same as that of theabove-described first embodiment. The operation of the transcodercontroller 3 is now explained.

[0139] The reception side transmission channel monitor 53 monitors thestate of the transmission channel over which the input bitstream istransmitted to output the sending out side reception channel information111 to the quantization step controller 73.

[0140] The sending out side transmission channel monitor 63 monitors thestate of the transmission channel over which the output bitstream istransmitted to output the sending out side transmission channelinformation 112 to the quantization step controller 73.

[0141] The input buffer monitor 52 monitors the storage volume of theinput buffer 21 of the decoder 1 to output the sending out sidetransmission channel information 112 to the quantization step controller73.

[0142] The output buffer monitor 62 monitors the storage volume of theoutput buffer 40 of the encoder 2 to output the output bufferinformation 109 to the quantization step controller 73.

[0143] The transcoder controller 3 determines the quantization stepinformation 103, based on the input buffer information 107 and theoutput buffer information 109, to deliver the resulting quantizationstep information 103 to the quantizer 33 of the encoder 2.

[0144] The reception side transmission channel information 111 and thesending out side transmission channel information 112 may also bereferenced in determining the quantization step information 103.

[0145] The quantization step controller 73 monitors the output bufferinformation 109, such that, if overflow would occur in the output buffer40, the quantization step information 103 is modified to reduce thevolume of the codes generated in the encoder 2 to prevent overflow fromoccurring in the output buffer 40.

[0146] On the other hand, if underflow would occur in the output buffer40, the quantization step information 103 is modified to increase thevolume of the codes generated in the encoder 2 to prevent underflow fromoccurring in the output buffer 40.

[0147] This prevents the failure in operation from occurring in theoutput buffer 40.

[0148] It is also possible for the quantization step controller 73 toset the target storage volume in the output buffer 40 to determine thequantization step information 103 from the relative magnitude of theoutput buffer information 109 as the actual storage volume and thetarget storage volume.

[0149] If the actual storage volume is larger than the target storagevolume, the quantization step information 103 is modified to decreasethe volume of the codes generated in the encoder 2.

[0150] That is, adjustment is made for increasing the quantization step.

[0151] If, conversely, the storage volume is smaller than the targetstorage volume, the quantization step information 103 is modified toincrease the code volume generated in the encoder 2.

[0152] That is, adjustment is made for decreasing the quantization step.

[0153] The target value can be variably set instead of being a fixedvalue.

[0154] It is also possible to set the upper and lower limit targetvalues and to set the target value within a certain preset range.

[0155] The quantization step controller 73 calculates the code volumeper processing unit (i.e., unit by unit) based on the input bufferinformation 107 and the output buffer information 109 in effectingpicture coding system conversion.

[0156] Now, considering the processing-unit-based pre- andpost-conversion code volumes in case of performing the picture encodingsystem conversion, if the pre-conversion code volume would become largerthan the post-conversion code volume, the band of the transmissionchannel transmitting the bitstream is not fully unilized.

[0157] If, conversely, the pre-conversion code volume is smaller thanthe post-conversion code volume, the bitstream cannot be sent with theband of the transmission channel sending out the bitstream, thus causingdelay at the output buffer.

[0158] That is, if the pre-conversion code volume is coincident with thepost-conversion code volume, the most effective utilization of the bandof the transmission channel and reduced time delay can be achieved.

[0159] The premise for this is that this is that the band of thetransmission channel for the input bitstream is controlled to be thesame as that of the transmission channel for the output bitstream.

[0160] That is, if the both bands would differ, the post-conversion codevolume is desirably coincident with the pre-conversion code volumemultiplied with a ratio R between the input transmission channel bandand the output transmission channel band.

[0161] The ratio R between the input transmission channel band and theoutput transmission channel band can be calculated from the receptionside transmission channel information 111 and the sending out sidetransmission channel information 112.

[0162] If both the input transmission channel and the outputtransmission channel are at the CBR (constant bit rate), that is thefixed transmission rate, the ratio R is constant, whereas, if one ofthem is at VBR (variable bit rate), that is at a variable transmissionrate, the ratio R is variable.

[0163] So, in the quantization step controller 73, effective utilizationof the transmission channel band and reduction in the delay time can berealized by determining the quantization step depending on thedifference between the processing-unit-based pre-conversion code volumeand the processing-unit-based post-conversion code volume as at the timeof performing the picture encoding system conversion.

[0164] For example, if the pre-conversion code volume is larger than thepost-conversion code volume, the quantization step information 103 ismodified such as to increase the code volume generated in the encoder 2at the time of the next processing.

[0165] If, conversely, the pre-conversion code volume is smaller thanthe post-conversion code volume, the quantization step information 103is modified such as to decrease the code volume generated in the encoder2 at the time of the next processing.

[0166] This allows to determine the quantization step per processingunit in converting the picture encoding system.

[0167] If the transmission channel is at the VBR, and there is certainallowance in the band of the transmission channel, the volume of thecodes generated in the encoder 2 may be increased to achieve theeffective band utilization.

[0168] If, conversely, there is no allowance in the band of thetransmission channel, the quantization step information 103 isdetermined such as to decrease the volume of the codes generated in theencoder 3.

[0169] As the method for modifying the quantization step, use may bemade of any of the conventional picture encoding systems, such as thequantization step modifying systems, e.g., H. 261, H. 263, or MPEG1, 2or 4.

[0170] A fourth embodiment of the present invention is now explained.FIG. 4 shows the configuration of the fourth embodiment of the presentinvention. Referring to FIG. 4, the fourth embodiment of the presentinvention includes, in addition to the components of the firstembodiment, a decoder monitor 51 for monitoring the reception sidetransmission channel, a reception side transmission channel monitor 53for monitoring the reception side transmission channel and a sending outside transmission channel monitor 63 for monitoring the sending out sidetransmission channel, these monitors 51, 53 and 63 belonging to thetranscoder controller 3. A quantization step controller 73 determinesthe quantization step information using the information output by theinput buffer monitor 52, output buffer monitor 62, decoder monitor 51,reception side transmission channel monitor 53 and the sending out sidetransmission channel monitor 63.

[0171] That is, referring to FIG. 4, the present third embodimentincludes decoder 1, encoder 2 and transcoder 3. The decoder 1 is made upof an input buffer 21, a VLD unit 22, an inverse-quantizer 23, an IDCTunit 24, an adder 25, a frame memory 26 and a motion compensationprediction unit 27. The encoder 2 is made up of an adder 31, a DCT unit32, a quantizer 33, an inverse-quantizer 34, an IDCT unit 35, an adder36, a frame memory unit 37, a motion compensation prediction unit 38, aVLC unit 39 and an output buffer 40. The transcoder controller 3 is madeup of a decoder monitor 51, an input buffer monitor 52, a reception sidetransmission channel monitor 53, a sending out side transmission channelmonitor 63, an output buffer monitor 62 and a quantization stepcontroller 74.

[0172] Since the operation of the decoder 1 is the same as that of thesecond embodiment, while the operation of the encoder 2 is the same asthat of the first embodiment, the corresponding explanation is omittedfor simplicity.

[0173] The operation of the transcoder controller 3 is now explained.

[0174] The reception side transmission channel monitor 53 monitors thestate of the transmission channel over which the input bitstream istransmitted to output the reception side transmission channelinformation 111 to the quantization step controller 74.

[0175] The sending out side transmission channel monitor 63 monitors thestate of the transmission channel over which the output bitstream istransmitted to output the sending out side transmission channelinformation 112 to the quantization step controller 74.

[0176] The decoder monitor 51 monitors the VLD unit 22 of the decoder 1to output the encoding parameters 108 to the quantization stepcontroller 72.

[0177] The input buffer monitor 52 monitors the storage volume of theinput buffer 21 of the decoder 1 to output the input buffer information107 to the quantization step controller 74.

[0178] The output buffer monitor 62 monitors the storage volume of theoutput buffer 40 of the encoder 2 to output the output bufferinformation 109 to the quantization step controller 74.

[0179] The transcoder controller 3 determines the quantization stepinformation 103, primarily based on the input buffer information 107 andthe output buffer information 109, to deliver the resulting quantizationstep information 103 to the quantizer (Q) 33 of the encoder 2.

[0180] The reception side transmission channel information 111 and thesending out side transmission channel information 112 may also bereferenced in determining the quantization step information 103.

[0181] The quantization step controller 74 monitors the output bufferinformation 109, such that, if overflow occurs in the output buffer 40,the quantization step information 103 is modified to reduce the volumeof the codes generated in the encoder 2 to prevent overflow fromoccurring in the output buffer 40.

[0182] On the other hand, if underflow occurs in the output buffer 40,the quantization step information 103 is modified to increase the volumeof the codes generated in the encoder 2 to prevent underflow fromoccurring in the output buffer 40.

[0183] This prevents the failure in operation from occurring in theoutput buffer 40.

[0184] It is also possible for the quantization step controller 74 toset the target storage volume in the output buffer 40 to determine thequantization step information 103 from the relative magnitude of theoutput buffer information 109 as the actual storage volume and thetarget storage volume.

[0185] If the actual storage volume is larger than the target storagevolume, the quantization step information 103 is modified such as todecrease the volume of the codes generated in the encoder 2.

[0186] If, conversely, the storage volume is smaller than the targetstorage volume, the quantization step information 103 is modified suchas to increase the code volume generated in the encoder 2.

[0187] The target value can be variably set instead of being a fixedvalue.

[0188] It is also possible to set the upper and lower limit targetvalues and to set the target value within a certain preset range.

[0189] The quantization step controller 74 is able to calculate the codevolume per processing unit based on the input buffer information 107 andthe output buffer information 109 in effecting picture coding systemconversion.

[0190] Now let's consider the processing-unit-based pre- andpost-conversion code volumes in case of performing the picture encodingsystem conversion. If the pre-conversion code volume would become largerthan the post-conversion code volume, the band of the transmissionchannel transmitting the bitstream would be not fully utilized.

[0191] If, conversely, the pre-conversion code volume is smaller thanthe post-conversion code volume, the bitstream cannot be sent with theband of the transmission channel sending out the bitstream, thus causingdelay at the output buffer.

[0192] That is, if the pre-conversion code volume is coincident with thepost-conversion code volume, the most effective utilization of the bandof the transmission channel and reduced time delay can be achieved.

[0193] The premise for this is that this is that the band of thetransmission channel for the input bitstream is the same as that of thetransmission channel for the output bitstream.

[0194] That is, if both the bands differ, the post-conversion codevolume should be desirably coincident with the pre-conversion codevolume multiplied with a ratio R between the input transmission channelband and the output transmission channel band.

[0195] The ratio R between the input transmission channel band and theoutput transmission channel band can be calculated from the receptionside transmission channel information 111 and the sending out sidetransmission channel information 112.

[0196] If both the input transmission channel and the outputtransmission channel are at the CBR (constant bit rate), that is thefixed transmission rate, the ratio R is constant, whereas, if one ofthem is at VBR (variable bit rate), that is at a variable transmissionrate, the ratio R is variable.

[0197] In the quantization step controller 74, effective utilization ofthe transmission channel band and reduction in the delay time can berealized by determining the quantization step depending on thedifference between the processing-unit-based pre-conversion code volumeand the processing-unit-based post-conversion code volume as at the timeof performing the picture encoding system conversion.

[0198] For example, if the pre-conversion code volume is larger than thepost-conversion code volume, the quantization step information 103 ismodified such as to increase the code volume generated in the encoder 2at the time of the next processing.

[0199] If, conversely, the pre-convers ion code volume is smaller thanthe post-conversion code volume, the quantization step information 103is modified such as to decrease the code volume generated in the encoder2 at the time of the next processing.

[0200] This allows to determine the quantization step per processingunit in converting the picture encoding system.

[0201] If the transmission channel is at the VBR, and there is certainallowance in the band of the transmission channel, the volume of thecodes generated in the encoder 2 may be increased to achieve theeffective band utilization.

[0202] If, conversely, there is no allowance in the band of thetransmission channel, the quantization step information 103 isdetermined such as to decrease the volume of the codes generated in theencoder 3.

[0203] The quantization step information 103 may also be determinedusing the encoding parameters 108 output from the decoder monitor indetermining the quantization step.

[0204] The reason is that, taking the picture quality following theconversion of the picture encoding system, the pre-conversion pictureproperties can be equated to the post-conversion picture properties byperforming the encoding using the quantization step obtained in thedecoder 1 in performing the quantization in the encoder 2.

[0205] If the quantization step used in decoding significantly differsfrom the quantization step used in encoding, picture distortion tends tobe produced.

[0206] The picture quality can be improved by determining thequantization step information 103 with the aid of the encodingparameters 108 output from the decoder monitor 51.

[0207] That is, control may be managed by performing fine adjustment,depending on the output buffer 40, pre-conversion code volume and on thepost-conversion code volume, using the quantization step output from thedecoder monitor as an initial value.

[0208] As the method for modifying the quantization step, use may bemade of any of the conventional picture encoding systems, such as thequantization step modifying systems, e. g., H. 261, H. 263, or MPEG1, 2or 4.

[0209]FIG. 9 is a flow diagram showing a typical operation of thequantization step controller 74 in the fourth embodiment of the presentinvention.

[0210] At step A1, the quantization step is provisionally determined bythe decoder operation information 102.

[0211] At step A2, the target code (generation) volume is set from theinput buffer information 107 based on the pre-conversion code volume perencoding system conversion processing unit.

[0212] At step A3, it is verified whether or not the stored volume ofthe output buffer is within the target range. If the stored volume iswithin the target range, the quantization step controller 74 proceeds tostep A4.

[0213] If the storage volume of the output buffer is verified at step A3to be outside the target range, the quantization step controller 74proceeds to step B1 top B4, depending on the storage volume.

[0214] At step A4, the quantization step Q is changed (modified)depending on the target code volume.

[0215] The quantization step information 103 is output at this time tothe quantizer 33 of the encoder 2.

[0216] At step A5, the encoder 2 performs encoding using thequantization step information 103 mentioned above.

[0217] At step A6, the volume of the actually produced codes isdetermined from the output buffer information 109 to update the targetcode volume of the generated codes.

[0218] If, at step A7, the end condition is not met, the processingreverts to step A1 making up a loop.

[0219] If, at step A3, the storage volume of the output buffer isoutside the target range (NO), processing branches to step B1(overflow), step B2 (the storage volume being larger than the lowertarget value), step B3 (the storage volume being smaller than the uppertarget value) or to step B4 (underflow), depending on the stored volumeof the output buffer. At steps B1 and B2, the processing transfers tostep C1 to increase the quantization step Q, whereas, at steps B3 andB4, the processing transfers to step C2 to decrease the quantizationstep Q before transferring to step A5.

[0220] A fifth embodiment of the present invention is hereinafterexplained. FIG. 5 shows the configuration of a code rate conversiondevice according to the fifth embodiment of the present invention.

[0221] Referring to FIG. 5, the fifth embodiment of the presentinvention includes a code rate conversion device 4 and a transcodercontroller 3. The code rate conversion device 4 is made up of an inputbuffer 21, a variable length decoding (VLD) unit 22, aninverse-quantizer (IQ) 84, an adder 85, an inverse discrete cosinetransform (IDCT) unit 86, a frame memory 87, a difference calculatingunit (ADDER) 88 and a discrete cosine transform (DCT) unit 89. Thetranscoder controller 3 is made up of an input buffer monitor 52, anoutput buffer monitor 62 and a quantization step controller 71.

[0222] Referring to FIG. 5, the operation of the code rate conversiondevice, according to the fifth embodiment of the present invention, isexplained in detail.

[0223] First, the operation of a code rate conversion device 5 isexplained.

[0224] The input buffer 21 stores the bitstream, supplied thereto fromoutside, to output the stored bitstream to the VLD unit 22.

[0225] On the other hand, the input buffer monitor 52 is fed with theinput buffer storage information 101.

[0226] The VLD unit 22 performs entropy decoding, such as variablelength decoding or run-length decoding, on the bitstream output from theinput buffer 21, to output decoded quantization transform coefficientsto the inverse-quantizer (IQ) 23.

[0227] The encoding parameters 201, such as the motion vector or theprediction mode, are sent to the difference calculating unit 88 and tothe VLC unit 83.

[0228] The inverse-quantizer 23 inverse-quantizes the quantizedtransform coefficients output from the VLD unit 22 to output theinverse-quantized transform coefficients.

[0229] The IDCT unit 24 performs IDCT matrix processing (calculations)coefficients on the transform coefficients output from the IQ unit 23 tooutput the transformed picture signals to the adder 81.

[0230] The adder 81 sums the signal output from the inverse-quantizer 23and an output of the DCT unit 89 to output a resulting sum to thequantizer (Q) 82 and to the adder 85.

[0231] The quantizer 82 quantizes the transform coefficients output fromthe adder 81 to output resulting quantized transform coefficients to theVLC unit 83 and to the inverse-quantizer (IQ) 84.

[0232] The VLC unit 83 performs entropy decoding, such as variablelength decoding or run-length decoding, on the quantized transformcoefficients output from the quantizer 82, and on the encodingparameters 201, output from the VLD unit 22, to output the resultingencoded signals to the output buffer 40.

[0233] The inverse-quantizer (IQ) 84 inverse-quantizes the quantizedtransform coefficients, output from the quantizer 82, to output theresulting transform coefficients to the adder 85.

[0234] The adder 85 sums the transform coefficients, output from theinverse-quantizer 84, and the transform coefficients output from theadder 81, more specifically, negative values thereof, to output aresulting sum to the IDCT unit 86.

[0235] The IDCT 86 performs IDCT matrix calculations on the transformcoefficients output from the adder 85 to output resulting picturesignals to the frame memory 87.

[0236] The frame memory 87 stores the picture signals output from theIDCT unit 86.

[0237] The difference calculating unit (ADDER) 88 calculates thedifference between a picture one frame before and the current picture tooutput the resulting difference to the DCT unit 89.

[0238] The DCT unit 89 performs DCT matrix calculations on thedifference data output from the difference calculating unit 88 to outputthe resulting transform coefficients to the adder 81.

[0239] The output buffer 40 stores the encoded signals, output from theVLC unit 83, to send out the encoded signals to outside the device.

[0240] The operation of the transcoder controller 3, having an inputbuffer monitor 52, an output buffer monitor 62 and a quantization stepcontroller 71, and configured for controlling the quantization step ofthe quantizer 82 by the quantization step controller 71, is similar tothat of the above-described first embodiment and hence is not explainedspecifically.

[0241] A sixth embodiment of the present invention is now explained.FIG. 6 shows the structure of a code rate conversion device according tothe sixth embodiment of the present invention. Referring to FIG. 6, thepresent sixth embodiment of the present invention includes, in additionto the structure of the above-described fifth embodiment, a decodermonitor 51, owned by the transcoder controller 3, for monitoring the VLDunit 22, with the quantization step controller 72 determining thequantization step information using the information output from theinput buffer monitor 52, output buffer monitor 62 and decoder monitor51. That is, the sixth embodiment of the present invention includes acode rate conversion device 4 and a transcoder controller 3. The coderate conversion device 4 is made up of an input buffer 21, a VLD unit22, an inverse-quantizer (IQ) 23, an adder 81, a quantizer 82, a VLCunit 83, an inverse-quantizer (IQ) 84, an adder 85, an IDCT unit 86, aframe memory 87, a difference calculating unit 88 and a DCT unit 89. Thetranscoder controller 3 is made up of a decoder monitor 51, an inputbuffer monitor 52, an output buffer monitor 62 and a quantization stepcontroller 73.

[0242] The operation of the code rate conversion device 4 is theoperation of the fifth embodiment plus the operation of the VLD unit 22outputting the encoding parameters 102 to the decoder monitor 51 of thetranscoder controller 3.

[0243] The operation of the transcoder controller 3 is similar to thatof the transcoder controller 3 in the above-described second embodimentand hence is not explained specifically.

[0244] A seventh embodiment of the present invention is now explained.FIG. 7 shows the structure of the code rate conversion device accordingto the seventh embodiment of the present invention. Referring to FIG. 7,the present seventh embodiment of the present invention includes, inaddition to the structure of the above-described fifth embodiment, areception side transmission channel monitor 53 for monitoring thereception side transmission channel, and a sending out side transmissionchannel monitor 63 for monitoring the sending transmission channel, bothowned by the transcoder controller 3, with the quantization stepcontroller 73 determining the quantization step information using theinformation output from the input buffer monitor 52, output buffermonitor 62, reception side transmission channel monitor 53 and thesending out side transmission channel monitor 63. That is, the sixthembodiment of the present invention includes a code rate conversiondevice 4 and a transcoder controller 3. The code rate conversion device4 is made up of an input buffer 21, a VLD unit 22, an inverse-quantizer(IQ) 23, an adder 81, a quantizer 82, a VLC unit 83, aninverse-quantizer (IQ) 84, an adder 85, an IDCT unit 86, a frame memory87, a difference calculating unit 88 and a DCT unit 89 The a transcodercontroller 3 is made up of an input buffer monitor 52, a reception sidetransmission channel monitor 53, an output buffer monitor 62, a sendingout side transmission channel monitor 63, and a quantization stepcontroller 73.

[0245] The operation of the code rate conversion device 4 and theoperation of the transcoder controller 3 are similar to those of thecode rate conversion device 5 in the previous fifth embodiment, and thetranscoder controller 3 in the previous third embodiment, respectively,and hence are not explained specifically.

[0246] An eighth embodiment of the present invention is now explained.FIG. 8 shows the structure of the code rate conversion device accordingto the eighth embodiment of the present invention. Referring to FIG. 8,the present eighth embodiment of the present invention includes, inaddition to the structure of the above-described fifth embodiment, adecoder monitor 51 for monitoring the VLD unit 22, a reception sidetransmission channel monitor 53 for monitoring the reception sidetransmission channel, and a sending out side transmission channelmonitor 63 for monitoring the sending transmission channel, thesemonitors 51, 53 and 63 being owned by the transcoder controller 3. Thequantization step controller 73 determines the quantization stepinformation using the information output from the input buffer monitor52, output buffer monitor 62, reception side transmission channelmonitor 53, decoder monitor 51 and the sending out side transmissionchannel monitor 63. That is, the present eighth embodiment of thepresent invention includes a code rate conversion device 4 and atranscoder controller 3. The code rate conversion device 4 is made up ofan input buffer 21, a VLD unit 22, an inverse-quantizer (IQ) 23, anadder 81, a quantizer 82, a VLC unit 83, an inverse-quantizer (IQ) 84,an adder 85, an IDCT unit 86, a frame memory 87, a differencecalculating unit 88 and a DCT unit 89. The transcoder controller 3 ismade up of an input buffer monitor 52, a reception side transmissionchannel monitor 53, an output buffer monitor 62, a sending out sidetransmission channel monitor 63, and a quantization step controller 73.

[0247] The operation of the code rate conversion device 4 and theoperation of the transcoder controller 3 are similar to those of thecode rate conversion device 5 in the previous sixth embodiment and thetranscoder controller 3 in the previous fourth embodiment, respectively,and hence are not explained specifically.

[0248] The meritorious effects of the present invention are summarizedas follows.

[0249] As described above, the present invention gives the followingmeritorious effects:

[0250] The first effect of the present invention is that, since the timedelay can be diminished, it is possible to realize encoding systemconversion and code rate conversion suited to real-time communication.

[0251] The second effect is that, since the quantization may be achievedby feedback of the pre-conversion and post-conversion code volumes, thetransmission channel band can be utilized most effectively, and the timedelay produced in conversion can be diminished.

[0252] The third effect is that control is managed not only from theencoding parameters obtained after VLD, but also from the input buffer,output buffer and code volume etc., and hence a more flexible conversionmay be achieved.

[0253] It should be noted that other objects, features and aspects ofthe present invention will become apparent in the entire disclosure andthat modifications may be done without departing the gist and scope ofthe present invention as disclosed herein and claimed as appendedherewith.

[0254] Also it should be noted that any combination of the disclosedand/or claimed elements, matters and/or items may fall under themodifications aforementioned.

What is claimed is:
 1. A picture encoding system conversion devicecomprising: a decoder receiving picture codes, compressed in informationvolume, from a reception side transmission channel via an input bufferand expanding the received picture codes to output expanded picturecodes; an encoder compressing the picture codes, decoded by saiddecoder, in information volume, to generate picture codes to outputgenerated picture codes from an output buffer to a sending sidetransmission channel; and a transcoder controller controlling saidencoder; wherein said transcoder controller includes an input buffermonitor monitoring said input buffer of said decoder; an output buffermonitor monitoring said output buffer of said encoder; and aquantization step controller modifying a quantization step incompression processing of said encoder based on the information fromsaid input buffer monitor and said output buffer monitor.
 2. The pictureencoding system conversion device as defined in claim 1 , wherein saiddecoder includes a variable length a decoder unit; said transcodercontroller further includes a decoder monitor monitoring said variablelength decoder unit; said quantization step controller modifying thequantization step of said encoder based on the information from saidinput buffer monitor, said output buffer monitor and the decodermonitor.
 3. The picture encoding system conversion device as defined inclaim 1 , wherein said transcoder controller includes a reception sidetransmission channel monitor monitoring the state of said receiving sidetransmission channel, and a sending side transmission channel monitormonitoring the state of said sending side transmission channel; saidquantization step controller modifying the quantization step of saidencoder based on the information from said input buffer monitor, saidoutput buffer monitor, the reception side transmission channel monitorand the sending side transmission channel monitor.
 4. The pictureencoding system conversion device as defined in claim 1 , wherein saiddecoder includes a variable length decoding unit; said transcodercontroller further includes a decoder monitor monitoring said variablelength decoding unit, a reception side transmission channel monitormonitoring the state of said reception side transmission channel, and asending side transmission channel monitor monitoring the state of saidsending side transmission channel; said quantization step controllermodifying the quantization step of said encoder based on the informationfrom said input buffer monitor, said output buffer monitor, said decodermonitor, the reception side transmission channel monitor and the sendingside transmission channel monitor.
 5. A code rate conversion devicecomprising: a code rate conversion unit receiving picture codescompressed in information volume from a reception side transmissionchannel by an input buffer, converting the code rate of the picturecodes and subsequently sending out the resultant picture codes throughan output buffer to a sending side transmission channel; and atranscoder controller controlling said code rate; wherein saidtranscoder controller includes an input buffer monitor monitoring saidinput buffer; output buffer monitor monitoring the output buffer; and aquantization step controller modifying a quantization step incompression processing of said code rate conversion unit based on theinformation from said input buffer monitor and said output buffermonitor.
 6. The code rate conversion device as defined in claim 5wherein said code rate conversion unit includes a variable lengthdecoding unit; said transcoder controller includes a decoder monitormonitoring said variable length decoding unit; said quantization stepcontroller modifying the quantization step of the code rate conversionunit based on the information from said input buffer monitor, saidoutput buffer monitor and said decoder monitor.
 7. The code rateconversion device as defined in claim 5 wherein said transcodercontroller further includes a reception side transmission channelmonitor monitoring the state of said reception side transmissionchannel; and a sending transmission channel monitor monitoring the stateof said sending out transmission channel; said quantization stepcontroller modifying the quantization step of the code rate conversionunit based on the information from said input buffer monitor, saidoutput buffer monitor, the reception side transmission channel monitorand the sending side transmission channel monitor.
 8. The code rateconversion device as defined in claim 5 wherein said code rateconversion unit includes a variable length decoding unit; saidtranscoder controller includes a decoder monitor monitoring saidvariable length decoding unit, a reception transmission channel monitormonitoring the state of said reception side transmission channel and asending transmission channel monitor monitoring the state of said asending out transmission channel; said quantization step controllermodifying a quantization step of said code rate conversion unit based onthe information from said input buffer monitor, said output buffermonitor, said decoder monitor, the reception side transmission channelmonitor and the sending side transmission channel monitor.
 9. Anencoding system conversion device for converting a signal encoded in oneencoding system into a signal of another encoding system, comprising: adecoder unit being fed with a compression-coded signal in an inputbuffer to decode the signal in a variable length decoder; an encoderunit being fed with a signal output from said decoder unit, having anorthogonal transform unit orthogonally transforming the input signal,having a quantizer quantizing coefficients resulting from the orthogonaltransform unit, and having a variable length encodercompression-encoding the quantized coefficients to output thecompression-coded quantized coefficients from an output buffer; and atranscoder controller having: means for monitoring at least the state ofsaid output buffer, and means for modifying a quantization step in saidquantizer (a) to decrease the code volume generated in said encoder ifthe stored volume in said output buffer exceeds a preset value toproduce overflow, and for modifying the quantization step in saidquantizer (b) to decrease the code volume generated in said encoder ifthe stored volume in said output buffer is not up to said preset valueto produce underflow.
 10. An encoding system conversion device forconverting a signal encoded in one encoding system into a signal ofanother encoding system, comprising: a decoder unit being fed with acompression-coded signal in an input buffer to decode the signal in avariable length decoder; an encoder unit being fed with a signal outputfrom said decoder unit, having an orthogonal transform unit orthogonallytransforming the input signal, having a quantizer quantizingcoefficients resulting from the orthogonal transformation, having avariable length encoder compression-encoding the quantized coefficientsand having an output buffer outputting compression-coded quantizedcoefficients to the sending side transmission channel; an input buffermonitor monitoring the state of said input buffer; an output buffermonitor monitoring the state of said output buffer; means for acquiringthe information of said reception side transmission channel and theinformation of said sending side transmission channel; and a transcodercontroller unit having a quantization step controller; said quantizationstep controller variably controlling a quantization step of saidquantizer, from the monitoring information for said input buffer and themonitoring information for said output buffer, based on the code volumeper processing unit upon converting the picture encoding system, saidvariably controlling being performed in a fashion: (a) that, if the bandof said reception side transmission channel is equal to that of saidsending side transmission channel, the pre-conversion code volume willbe equal to the post-conversion code volume, and (b) that, if the bandof said reception side transmission channel is different from that ofsaid sending side transmission channel, the post-conversion code volumewill coincide with the pre-conversion code volume multiplied with aratio between bands of said reception side transmission channel and saidsending side transmission channel.
 11. The encoding system conversiondevice as defined in claim 9 wherein said transcoder controller unitfurther includes a decoder monitor fed with encoding parameters outputfrom said variable length decoder; said quantization step controllerdetermining the quantization step of said quantizer using said encodingparameters.
 12. The encoding system conversion device as defined inclaim 10 wherein said transcoder controller unit further includes adecoder monitor fed with encoding parameters output from said variablelength decoder; said quantization step controller determining thequantization step of said quantizer using said encoding parameters. 13.The encoding system conversion device as defined in claim 11 wherein thequantization step of said quantizer is finely adjusted, with thequantization step output from said decoder monitor, as an initial value,responsive to the state of said output buffer, pre-conversion codevolume and said post-conversion code volume.
 14. The encoding systemconversion device as defined in claim 12 wherein the quantization stepof said quantizer is finely adjusted, with the quantization step outputfrom said decoder monitor, as an initial value, responsive to the stateof said output buffer, pre-conversion code volume and saidpost-conversion code volume.
 15. A code rate conversion device forconverting a signal encoded in one encoding system into a signal ofanother encoding system, comprising: a code rate conversion unit and atranscoder controller; said code rate conversion unit at leastincluding: an input buffer fed from a reception side transmissionchannel with a signal compressed in information volume, a variablelength decoder decoding the signal, an inverse quantizerinverse-quantizing an output of said variable length decoder, an adderdirectly outputting an output of said inverse quantizer or outputting avalue corresponding to said inverse quantizer output minus an orthogonaltransformed difference between a current picture and a preceding framepicture, a quantizer quantizing an output of said adder, and a variablelength decoder, said code rate conversion unit sending out a code rateconverted signal from the output buffer; and said transcoder controllerhaving: an output buffer monitor monitoring at least the state of saidoutput buffer, a quantization step modifier modifying a quantizationstep in said quantizer to decrease generating code volume if the storedvolume in said output buffer exceeds a preset value resulting inoverflow, said modifier modifying the quantization step in saidquantizer to decrease generating code volume if the stored volume insaid output buffer is not up to said preset value resulting inunderflow.
 16. A code rate conversion device for converting a signalencoded in one encoding system into a signal of another encoding system,comprising: (a) a code rate conversion unit at least including: (a1) aninput buffer fed from a reception side transmission channel with asignal compressed in information volume, (a2) a variable length decoderdecoding the signal, (a3) an inverse quantizer inverse-quantizing anoutput of said variable length decoder, (a4) an adder directlyoutputting an output of said inverse quantizer or outputting a valuecorresponding to said inverse quantizer output minus an orthogonaltransformed difference between a current picture and a preceding framepicture, (a5) a quantizer quantizing an output of said adder, and (a6) avariable length decoder, said code rate conversion unit sending out acode rate converted signal from the output buffer; (b) an input buffermonitor monitoring the state of said input buffer; (c) an output buffermonitor monitoring the state of said output buffer; (d) means foracquiring information on said reception side transmission channel andinformation on said sending side transmission channel; and (e) atranscoder controller having a quantization step controller; saidquantization step controller variably controlling a quantization step ofsaid quantizer, from the monitoring information for said input bufferand the monitoring information for said output buffer, based on the codevolume per processing unit in converting the picture encoding system, sothat, if a band of said reception side transmission channel is equal tothat of said sending side transmission channel, the pre-conversion codevolume will be equal to the post-conversion code volume, and so that, ifa band of said reception side transmission channel is different fromthat of said sending side transmission channel, the post-conversion codevolume will coincide with the pre-conversion code volume multiplied witha ratio between the band of said reception side transmission channel andthe band of said sending side transmission channel.
 17. The code rateconversion device as defined in claim 15 wherein said transcodercontroller further includes a decoder monitor having encoding parametersoutput from said variable length decoder as an input; said quantizationstep controller determining the quantization step of said quantizerusing said encoding parameters.
 18. The code rate conversion device asdefined in claim 16 wherein said transcoder controller further includesa decoder monitor having encoding parameters output from said variablelength decoder as an input; said quantization step controllerdetermining the quantization step of said quantizer using said encodingparameters.
 19. A picture code rate conversion device comprising: (a) adecoder, (b) an encoder and (c) a transcoder controller; said decoder(a) including: (a1) an input buffer unit receiving compression-codedpicture codes from a reception side transmission channel; (a2) avariable length decoder decoding picture codes of said input bufferunit; (a3) a first IDCT unit inverse-discrete-cosine-transforming anoutput of said variable length decoder; (a4) a first adder on one inputend of which an output of said first IDCT unit is fed; (a5) a firstframe memory unit receiving and memorizing an output of said firstadder; and (a6) a first motion compensation prediction unit receiving anoutput of said variable length decoder and an output of said first framememory unit; said encoder (b) including (b1) a second adder receiving,at one input end thereof, an output of said decoder; (b2) a DCT unitdiscrete-cosine-transforming an output of said second adder; (b3) aquantizer quantizing an output of said DCT unit; (b4) a variable-lengthencoder receiving an output of said quantizer; (b5) an output bufferreceiving an output of said variable length decoder to output thisoutput to a sending side transmission channel; (b6) a second inversequantizer inverse-quantizing an output of said quantizer; (b7) a secondIDCT unit inverse-discrete-cosine-transforming an output of said secondinverse quantizer; (b8) a third adder receiving, at an input endthereof, an output of said second IDCT unit; (b9) a second frame memoryunit receiving and memorizing an output of said third adder; and (b10) asecond motion compensation prediction receiving an output of said secondframe memory unit and an output of said first motion compensationprediction unit; (b11) an output of said second motion compensationprediction unit being fed to the other input end of said second andthird adder; and said transcoder (c) controller including: (c1) anencoder receiving, at the other input ends of said second and thirdadders thereof, an output of said second motion compensation predictionunit; (c2) an input buffer monitor monitoring said input buffer; outputbuffer monitor monitoring said output buffer; and a quantization stepcontroller variably controlling a quantization step of said quantizer ofsaid encoder based on the monitoring information output from said inputbuffer monitor and said output buffer monitor.
 20. The picture code rateconversion device as defined in claim 19 wherein said transcodercontroller further includes at least one selected from the groupconsisting of a reception channel monitor monitoring one of saidreception side transmission channel, a sending channel monitormonitoring said sending side transmission channel and a variable lengthdecoder monitor receiving encoding parameters output from said variablelength decoder to monitor said variable length decoder; saidquantization step controller variably controlling the quantization stepof said quantizer of said encoder based on the monitoring informationoutput from said respective monitors.
 21. A picture code rate conversiondevice comprising: (a) a coding rate conversion unit and (b) atranscoder controller unit; said coding rate conversion unit (a)including: (a1) an input buffer unit receiving a signal from a receptionside transmission channel; (a2) a variable length decoding unit decodingpicture codes of said input buffer unit; (a3) a first IDCT unitinverse-discrete-cosine-transforming an output of said first variablelength decoder; (a4) a first adder on one input end of which an outputof said first IDCT unit is fed; (a5) a quantizer quantizing an output ofsaid first adder; (a6) a variable length encoder encoding an output ofsaid quantizer to output an encoded signal; (a7) an output bufferreceiving the encoded signal from said variable length encoder to outputa resulting output to a sending side transmission channel; (a8) a secondinverse quantizer inverse-quantizing an output of said quantizer; (a9) asecond adder subtracting an output of said first adder from an output ofsaid second inverse quantizer to output a resulting difference signal;(a10) an IDCT unit receiving an output of said second adder as an input;(a11) a frame memory unit receiving and storing an output of said IDCTunit; (a12) a difference calculating unit taking a difference between acurrent picture output from said variable lenth decoding unit and apicture one frame before from said frame memory unit; and (a13) a DCTunit receiving an output of said difference calculating unit as aninput; (a14) wherein said first adder outputs a value obtained bysubtracting an output of said IDCT from an output of said first inversequantizing unit; said transcoder controller unit (b) including: (b1) aninput buffer monitor monitoring said input buffer; (b2) an output buffermonitor monitoring said output buffer; and (b3) quantization stepcontroller variably controlling a quantization step of said quantizer ofsaid encoder based on the monitoring information output from said inputbuffer monitor and said output buffer monitor.
 22. The code rateconversion device as defined in claim 18 wherein said transcodercontroller unit further includes a reception channel monitor monitoringone of said reception side transmission channel, a said reception sidetransmission channel, a sending channel monitor monitoring said sendingside transmission channel, and a variable length decoder receivingencoding parameters output from said variable length decoder to monitorsaid variable length decoder; said quantization step controller variablycontrolling a quantization step of said quantizer of said encoder basedon the monitoring information output from said respective monitors.